The present invention relates to a pressure switch, particularly, to a pressure switch for use in a sensor of a device for alarming the reduction of pneumatic pressure of vehicle tire.
Such a pressure switch used in a sensor provided in the device for alarming the reduction of pneumatic pressure of vehicle tire, is constructed as shown in FIG. 5. That is, a diaphragm 1 and a contact rod 3 having one contact 2 provided on the tip thereof are secured to a holder or supporting member 4 so as to form or define a back pressure chamber 5, and then gas is sealed in the back pressure chamber 5 from a back pressure sealed inlet 6 until normal pressure (usually the same pressure as the pneumatic pressure of a tire, for example, 2.5 kg/cm.sup.2) is obtained, and then the sealed inlet 6 is sealed and welded with a sealing steel ball 7. The contact rod 3 and the holder 4 are secured in a housing 8 which is also co-used as a hexagon nut, and then epoxy resin 19 is filled in the housing 8. The housing 8 is, then, threaded on a housing 11 by an O-ring 12 in which an oscillating coil or a resonance coil 9 and a capacitor 10 are accommodated. The resonance coil 9 is wound on a core 13 of magnetic substance.
In this case, a pin hole is liable to result in a welded portion 14 of the diaphragm 1 and the supporting member 4, and thus the gas pressure in the back pressure chamber 5 becomes equal to the air pressure in the wheel tire, so that the pressure switch is maintained in an opened condition or closed condition.
When a pin hole is caused at the welded portion of the sealing steel ball 7 and the sealed inlet 6 of the supporting member 4, the gas is liable to leak through the epoxy resin 19 or a side wall 15 of the housing 8. The side wall 15 is provided with two O-rings 12 at separate portions thereof, but the sealing with these O-rings is not sufficient, since a slight gas leak results in a large pressure loss due to small capacity of the back pressure chamber 5.
In order to improve the detecting capability of pressure difference by the diaphragm 1, moreover, it can not avoid to make the diameter of the diaphragm 1 large, so that if the diameter of the diaphragm 1 is 15 mm-20 mm, the diameter of threaded portion provided on the surface of the side wall 15 is 30 mm-35 mm or more.
Since in the latter case, the pressure of the back pressure chamber 5 is decreased, the precision of detecting the reduction of pneumatic pressure of a vehicle tire after fabrication gradually decreased. That is, when the pneumatic pressure of the vehicle tire is at a normal pressure, 2.5 kg/cm.sup.2, gas pressure of the back pressure chamber 5 also becomes substantially 2.5 kg/cm.sup.2, so that the contact 2 is always contacted to the diaphragm 1. If the pneumatic pressure of tire is decreased by 0.5 kg/cm.sup.2, the contact 2 is released, resulting in a switched off condition so that the alarming device generates an alarm together with other circuits. In this case, when the operating pressure of the diaphragm 1 is 2.0 kg/cm.sup.2, and the pressure of the back pressure chamber 5 is decreased by 0.5 kg/cm.sup.2, then the switch does not switch to the off state, and the alarm does not occur as long as the pneumatic pressure of the vehicle tire is not decreased to 1.5 kg/cm.sup.2.
In order to eliminate such a disadvantage, that is, in order to prevent secular change of detection capability, or to perform small size, light weight and inexpensive price, a pressure switch shown in FIG. 6 has been provided. That is, a cup-shaped diaphragm 21 is placed on the bottom of a housing 20 having a step portion and these components are welded to form a hermetic chamber. A contact 22 is provided to the center portion of the diaphragm 21. A ceramic contact rod supporting plate 25 is provided with a female screw bush 23 at the center thereof. A screw contact rod 24 is adjustably inserted in the bush 23. The supporting plate 25 is inserted in the housing 20 and placed and fixed on and to the step portion thereof, to define a hermetic chamber 26 corresponding to the back pressure chamber and then epoxy resin 27 is filled on to the supporting plate 25, thereby forming a pressure switch. Since in the pressure switch shown in FIG. 5 the back pressure switch has a gas pressure which is the same as the pneumatic pressure of the tire which is higher than atmospheric pressure thereby causing a problem, the back pressure chamber, that is, the hermetic chamber 26 of the pressure switch shown in FIG. 6 is the same pressure as the atmospheric pressure. In the pressure switch shown in FIG. 6, in order to prevent inflow and outflow of air due to expansion (pressure increase) and reduction (pressure decrease) of air in the corresponding back pressure chamber with variations in atmospheric temperature, resin such as epoxy is injected and fixed on and to the supporting plate 25 for the contact rod 23. In this case, when the atmospheric temperature is higher than that of the pressure switch during manufacture, the expanded air in the hermetic chamber 26 leaks and then when the atmospheric temperature is lower than the above temperature or equal to the normal temperature, the air in the hermetic chamber 26 becomes lower than the atmospheric pressure and thus the detecting precision becomes decreased.
As described above, in the conventional pressure switch, when gas is sealed in the hermetic chamber or back pressure chamber to obtain a suitable back pressure, and then to cause the switch pressure to be substantially proportional to the absolute temperature, gas is leaked outwardly through the cover member or substance of epoxy resin, so that the switch pressure-temperature characteristic can not be maintained for long period.
When the pressure switch is used in the device for alarming the reduction of pneumatic pressure of a vehicle tire, it is desirable to use a small pressure switch. Even if, in the conventional diaphragm system, the size of the diaphragm must be made small in order to obtain a small pressure switch, however, the displacement of the diaphragm becomes very small, so that the precision of pressure detection decreased and thus the pressure switch can not be made small.
The diaphragm can not be fabricated with continuous thickness, so that any switch pressure characteristic can not be obtained.